U.S. patent application number 12/787570 was filed with the patent office on 2010-12-02 for stable dispersions of single and multiple graphene layers in solution.
This patent application is currently assigned to BELENOS CLEAN POWER HOLDING AG. Invention is credited to Reinhard Nesper, Kaspar Tommy.
Application Number | 20100301279 12/787570 |
Document ID | / |
Family ID | 41328830 |
Filed Date | 2010-12-02 |
United States Patent
Application |
20100301279 |
Kind Code |
A1 |
Nesper; Reinhard ; et
al. |
December 2, 2010 |
STABLE DISPERSIONS OF SINGLE AND MULTIPLE GRAPHENE LAYERS IN
SOLUTION
Abstract
Disclosed is a method for producing colloidal graphene
dispersions comprising the steps of (i) dispersing graphite oxide
in a dispersion medium to form a colloidal graphene oxide or
multi-graphene oxide dispersion (ii) thermally reducing the
graphene oxide or multi-graphene oxide in dispersion. Dependent on
the method used for the preparation of the starting dispersion a
graphene or a multi-graphene dispersion is obtained that can be
further processed to multi-graphene with larger inter-planar
distances than graphite. Such dispersions and multi-graphenes are
for example suitable materials in the manufacturing of rechargeable
lithium ion batteries.
Inventors: |
Nesper; Reinhard; (Amden,
CH) ; Tommy; Kaspar; (Mels, CH) |
Correspondence
Address: |
GRIFFIN & SZIPL, PC
SUITE PH-1, 2300 NINTH STREET, SOUTH
ARLINGTON
VA
22204
US
|
Assignee: |
BELENOS CLEAN POWER HOLDING
AG
Bienne
CH
|
Family ID: |
41328830 |
Appl. No.: |
12/787570 |
Filed: |
May 26, 2010 |
Current U.S.
Class: |
252/502 ;
427/122 |
Current CPC
Class: |
H01B 1/04 20130101; H01B
13/32 20130101; C01B 32/192 20170801; H01M 4/587 20130101; C01B
2204/30 20130101; C01B 2204/02 20130101; C01B 2204/28 20130101;
B82Y 30/00 20130101; B82Y 40/00 20130101; H01M 10/052 20130101;
Y10T 428/30 20150115; Y02E 60/10 20130101; Y02E 60/32 20130101;
Y02P 70/50 20151101 |
Class at
Publication: |
252/502 ;
427/122 |
International
Class: |
H01B 1/04 20060101
H01B001/04; B05D 5/12 20060101 B05D005/12 |
Foreign Application Data
Date |
Code |
Application Number |
May 26, 2009 |
EP |
09161106.1 |
Claims
1. A method for producing colloidal graphene dispersions or
multi-graphene dispersions, the method comprising the steps of: (i)
dispersing graphite oxide in a dispersion medium to form a
colloidal graphene oxide dispersion or a multi-graphene oxide
dispersion; and (ii) thermally reducing the graphene oxide or
multi-graphene oxide in dispersion to form a colloidal graphene
dispersion or a multi-graphene dispersion.
2. The method of claim 1 wherein step (ii) is performed at a
temperature of at least 120.degree. C.
3. The method of claim 1 wherein step (ii) is performed at a
temperature of 120.degree. C. to 130.degree. C.
4. The method of claim 1 wherein step (ii) is performed at a
temperature of at least 130.degree. C.
5. The method of claim 1 wherein step (ii) is performed at a
temperature of at least 150.degree. C.
6. The method of claim 1, wherein the dispersion medium is a
protic, polar dispersion medium, preferably a water based
dispersion medium.
7. The method of claim 6, wherein when the dispersion medium is
water, the dispersion produced in step (i) is a colloidal graphene
oxide dispersion and the dispersion produced in step (ii) is a
colloidal graphene dispersion.
8. The method of claim 6, wherein the dispersion medium comprises
ammonia.
9. The method of claim 1, wherein the dispersion medium is an
aprotic, polar dispersion medium, preferably acetonitrile.
10. The method of claim 1, wherein step (ii) is performed at a
temperature and for a time to result in a colloidal graphene
dispersion with a carbon:oxygen (C/O) ratio of between about 4 and
about 25.
11. The method of claim 6, wherein when the dispersion medium is
acidified water, preferably water of a pH of about 4, the
dispersion produced in step (i) is a multi-graphene oxide
dispersion and the dispersion produced in step (ii) is a
multi-graphene dispersion.
12. A method for producing a graphene layer using a colloidal
graphene or multi-graphene layers using a multi-graphene
dispersion, the colloidal graphene or the multi-graphene
dispersion, respectively, obtained by the method of claim 1
comprising the steps of: (i) dispersing graphite oxide in a
dispersion medium to form a colloidal graphene oxide or a
multi-graphene oxide dispersion; and (ii) thermally reducing the
graphene oxide or multi-graphene oxide in dispersion to form a
colloidal graphene dispersion or a multi-graphene dispersion, the
method for producing a graphene layer or multi-graphene layers,
comprising, either the steps of: (1) deposit the dispersion of step
(i) on a substrate or precipitate the dispersion of step (i); and
(2) then perform step or the step of deposit the dispersion of step
(ii) on a substrate or precipitate the dispersion of step (ii).
13. A colloidal graphene dispersion obtained by the method of claim
1, the colloidal graphene dispersion having a C/O ratio of about 4
and about 25.
14. The colloidal graphene dispersion, according to claim 13, the
colloidal graphene dispersion having a graphene content of 0.1% to
0.5% by weight.
15. The colloidal graphene dispersion according to claim 13 wherein
the colloidal graphene dispersion does not comprise any
dispersant.
16. A multi-graphene dispersion obtained by the method of claim 1,
the multi-graphene dispersion having an inter-planar distance, in
the multi-graphene dispersion, of greater than 3.35 .ANG..
17-19. (canceled)
20. A method for generating electronically conducting nanoparticles
using the colloidal graphene dispersion according to claim 13, the
method comprising the steps of: (a-1) coating nanoparticles with
the colloidal graphene dispersion; and (a-2) then thermally
reducing said coated particles in dispersion to get nanoparticles
coated with graphene/multi-graphene of a C/O ratio of at least
13.5.
21. A method for preparing an electrode for rechargeable lithium
ion batteries using the multi-graphene dispersion according to
claim 16, the method comprising the step of: (b-1) providing the
multi-graphene dispersion; and (b-2) coating a conductor with said
multi-graphene dispersion, optionally, in the presence of a
binder.
22. A method for preparing an electrode for rechargeable lithium
ion batteries using the multi-graphene layers according to claim
39, the method comprising the steps of: (c-1) providing the
multi-graphene layers, wherein the multi-graphene layers have low
C/O ratio of at most 13; and (c-2) coating a conductor with said
multi-graphene layers, optionally, in the presence of a binder.
23. The method of claim 1, wherein step (ii) is performed at a
temperature and for a time to result in a colloidal graphene
dispersion with a carbon:oxygen (C/O) ratio of between about 4 and
about 20.
24. The method of claim 1, wherein step (ii) is performed at a
temperature and for a time to result in a colloidal graphene
dispersion with a carbon:oxygen (C/O) ratio of between about 4 and
about 13.5.
24. The method of claim 1, wherein step (ii) is performed at a
temperature and for a time to result in a colloidal graphene
dispersion with a carbon:oxygen (C/O) ratio of about 7.
25. The colloidal graphene dispersion obtained by the method of
claim 1, the colloidal graphene dispersion having a C/O ratio of
between about 4 and about 20.
26. The colloidal graphene dispersion obtained by the method of
claim 1, the colloidal graphene dispersion having a C/O ratio of
between about 4 and about 13.5.
27. The colloidal graphene dispersion obtained by the method of
claim 1, the colloidal graphene dispersion having a C/O ratio of
about 7.
28. The colloidal graphene dispersion according to claim 13, the
colloidal graphene dispersion having a graphene content of about
0.1% by weight.
30. The colloidal graphene dispersion according to claim 25, the
colloidal graphene dispersion having a graphene content of 0.1% to
0.5% by weight.
31. The colloidal graphene dispersion according to claim 26, the
colloidal graphene dispersion having a graphene content of 0.1% to
0.5% by weight.
32. The colloidal graphene dispersion according to claim 27, the
colloidal graphene dispersion having a graphene content of 0.1% to
0.5% by weight.
33. The colloidal graphene dispersion according to claim 25,
wherein the colloidal graphene dispersion does not comprise any
dispersant.
34. The colloidal graphene dispersion according to claim 26,
wherein the colloidal graphene dispersion does not comprise any
dispersant.
35. The colloidal graphene dispersion according to claim 27,
wherein the colloidal graphene dispersion does not comprise any
dispersant.
36. A multi-graphene dispersion obtained by the method of claim 1,
the multi-graphene dispersion having an inter-planar distance, in
the multi-graphene dispersion, of greater than 3.40 .ANG..
37. A multi-graphene dispersion obtained by the method of claim 1,
the multi-graphene dispersion having an inter-planar distance, in
the multi-graphene dispersion, of greater than 3.50 .ANG..
38. A multi-graphene dispersion obtained by the method of claim 1,
the multi-graphene dispersion having an inter-planar distance, in
the multi-graphene dispersion, of greater than 3.60 .ANG..
39. Multi-graphene layers obtained by the method of claim 12, the
multi-graphene layers having an inter-planar distance, in the
multi-graphene layers, of greater than 3.35 .ANG..
40. Multi-graphene layers obtained by the method of claim 12, the
multi-graphene layers having an inter-planar distance, in the
multi-graphene layers, of greater than 3.40 .ANG..
41. Multi-graphene layers obtained by the method of claim 12, the
multi-graphene layers having an inter-planar distance, in the
multi-graphene layers, of greater than 3.50 .ANG..
42. Multi-graphene layers obtained by the method of claim 12, the
multi-graphene layers having an inter-planar distance, in the
multi-graphene layers, of greater than 3.60 .ANG..
43. A method for generating electronically conducting nanoparticles
using a colloidal graphene oxide dispersion obtained by the method
of claim 1, comprising the steps of: (d-1) coating nanoparticles
with the colloidal graphene oxide dispersion; and (d-2) then
thermally reducing said coated particles in dispersion to get
nanoparticles coated with graphene/multi-graphene of a C/O ratio of
at least 13.5.
Description
TECHNICAL FIELD
[0001] The present invention concerns the field of electronically
conductive carbonaceous materials and their production, in
particular materials suitable for use in rechargeable lithium ion
batteries.
BACKGROUND ART
[0002] The term graphene designates a one atom thick planar sheet
of sp.sup.2-hybridized rings with 6 carbon atoms. Perfect graphenes
consist exclusively of hexagonal cells. Cylindrical graphene layers
are termed carbon nanotubes. The term graphene may also be used
when features of single layers in graphite are discussed. Such
features are e.g. reactivity or undergone reactions, respectively,
or structural relations.
[0003] Graphene layers may be produced by suitable abrasion,
mechanical exfoliation or chemical vapour deposition. One such
method is the so called Scotch-Tape-Method [10]. In this method
single layers are removed from a graphite crystal and transferred
to a sample holder.
[0004] In a recently described chemical vapour deposition method is
disclosed, wherein, prior to being able to generate a graphene
layer, a SiO.sub.2/Si substrate has to be covered with a thin Ni
layer, and this coated substrate has to be subjected to a specific
gas treatment. For making the graphene layer usable to coat other
substrates, the Ni or the SiO.sub.2 layer have to be dissolved.
Such graphene films had very good electrical, optical and
mechanical (e.g. bending) properties [9].
[0005] Another method is to heat silicon carbide to high
temperatures (1100.degree. C.) to reduce it to graphene. This
process produces a layer the extent of which is dependent on the
size of the SiC substrate used and--due to the expensive starting
material--is quite expensive and also limited in use due to the
high temperature needed.
[0006] Graphene is quite different from most solids. Graphene
behaves as a semi-metal or "zero-gap" semiconductor and has a
remarkably high electron mobility at room temperature.
[0007] Aqueous dispersions of carbonaceous material such as
graphite, graphene or carbon nanotubes are described in literature.
The production of aqueous graphite dispersions from graphite with
preferred particle sizes between 1 .mu.m and 50 .mu.m or 100 .mu.m,
respectively, stabilized by various dispersants has been described
(see e.g. U.S. Pat. No. 5,476,580 and WO2007/031055). Up to 20% by
weight or up to 70% by weight, respectively, graphite may be
dispersed in water.
[0008] Dispersions of carbon nanotubes with a nanotubes content of
2% are e.g. obtainable by stabilization with the dispersing aid
polyethylene glycol [1] or via chemical functionalizing of the
carbon nanotubes [2].
[0009] WO2008/048295 describes a method for stabilizing graphene
layers in a solvent by means of polymer coating. An about 0.065% by
weight graphene based material is obtained. The colloidal graphene
dispersion is provided by reduction of dispersed graphite oxide
using hydrazine hydrate.
[0010] Dan Li et al. [3] describe that the aqueous solution may be
electrostatically stabilized by ammonia resulting in a graphene
based material with graphene content of about 0.015% by weight.
Also Dan Li et al. prepared the colloidal graphene dispersion from
a graphite oxide dispersion by reduction with hydrazine hydrate.
The reduction with hydrazine hydrate as disclosed in the state of
the art results in a C/O ratio of below 13.5 meaning that at most
about 80% of the oxygen have been removed [4, 5, 6].
[0011] Another method for reduction of graphite oxide is thermal
reduction. Dependent on the desired production conditions, purity
conditions and reduction conditions, the thermal reduction of
graphite oxide powder is slow up to a temperature of about
200.degree. C. and then becomes boisterous [7]. Reduction at this
temperature results in an elimination of about 65% of the oxygen,
10% of the carbon and most of the hydrogen due to the formation of
CO, CO.sub.2 and water. Heating to higher temperatures results in
continuous further reduction. A temperature of about 1000.degree.
C. is required for removal of about 90% of the oxygen. Thus
produced graphite material can no longer be dispersed in water to
form a colloidal dispersion.
DISCLOSURE OF THE INVENTION
[0012] Hence, it is a general object of the invention to provide a
method for producing stable colloidal dispersions of graphene in
solution, in particular colloidal graphene dispersions that do not
need any dispersant.
[0013] It is also an object of the present invention to provide
stable dispersions of single and multiple graphene layers.
[0014] It is also an object of this invention to provide uses of
such graphene dispersions.
[0015] It is a further object of the invention to provide a
multi-graphene with improved intercalating features that may e.g.
advantageously be used for the production of electrodes in
rechargeable lithium ion batteries.
[0016] Now, in order to implement these and still further objects
of the invention, which will become more readily apparent as the
description proceeds, the method for producing colloidal graphene
dispersions is manifested by the features that it comprises the
steps of
[0017] (i) dispersing graphite oxide in a dispersion medium to form
a colloidal graphene oxide or graphite oxide dispersion,
[0018] (ii) thermally reducing the graphene oxide or graphite oxide
in dispersion.
[0019] In the scope of the present invention the dispersion related
terms are used with the following meanings:
[0020] Dispersion is a two phase system consisting of a disperse
phase in a dispersion medium, wherein the disperse phase is finely
divided in the dispersion medium.
[0021] Disperse phase designates the solid, optionally colloidal
phase.
[0022] Dispersion medium is used as synonym to continuous phase and
liquid phase. Since the dispersion medium is a substance or a
mixture of substances known as solvents, also the term solvent or
solvent phase is used. The dispersion medium may also comprise
additives and adjuvants such as dispersants.
[0023] Dispersants are substances that assist in stabilizing
dispersions, that e.g. prevent coagulation and/or aggregation of
the dispersed phase, such as surfactants. In this text, dispersants
are also termed dispersing agents, dispersing aids.
[0024] In the scope of the present invention, the terms describing
the carbonaceous material are used as follows:
[0025] If not further specified, the term graphene alone or in
graphene oxide designates a single layer of graphite or graphite
oxide, respectively.
[0026] An assembly of two or more graphene layers with the usual
inter-planar distance of graphite are termed graphite.
[0027] An assembly of two or more graphene layers with a larger
inter-planar distance than in graphite are termed
multi-graphene.
[0028] For graphite oxide the same distinction is made, in
particular with regard to the product finally obtained
therefrom.
[0029] The terms graphene/multi-graphene/graphite are used for
carbonaceous materials with carbon:oxygen (C/O) ratio of 4, the
terms graphene oxide/multi-graphene oxide/graphite oxide for
materials with C/O ratio <4
[0030] Reducing the graphene oxide in dispersion means that during
the reduction step the graphene oxide/graphene or graphite
oxide/graphite remains dispersed in solvent. For low boiling
solvents, this can be achieved by heating the dispersion under
pressure, e.g. in an autoclave, preferably an autoclave with a
polytetrafluoroethylen (Teflon.RTM.) insert or a glass
autoclave.
[0031] Dependent on the desired carbon:oxygen (C/O) ratio, the
thermal reduction step is performed at low temperatures of about
120 to 130.degree. C. or at higher temperatures of at least about
130.degree. C. Temperatures around 120.degree. C. allow fine-tuning
of the desired C/O ratio but they cannot be used for reduction up
to high C/O ratios or only at very long reaction times. The higher
the reaction temperature, the faster the reduction proceeds. This
allows the fast production of high C/O ratios.
[0032] For example, by the inventive method, already at 140.degree.
C. about 80 of the oxygen are removed and thus already at this
temperature a C/O ratio of above 13.5 is obtained, i.e. a C/O ratio
better than reported for reduction with hydrazine hydrate. Already
at 250.degree. C. (compared with the 1000.degree. C. reported in
the literature) more than 90% of the oxygen is removed while the
colloidal dispersion of the single graphene layers is still
longtime stable.
[0033] Longtime stable in the context of the present invention
means that the graphene remains colloidally dispersed for at least
1 day, preferably for at least 1 week, more preferred for at least
1 month or 3 months, most preferred for at least 1 year (measured
at room temperature for a 0.1% by weight dispersion).
[0034] Colloidal graphene dispersions of the present invention are
preferably produced with a graphene content of at most 0.5% by
weight, usually about 0.1% by weight or--dependent on the
application--more diluted. Such stable dispersions may be obtained
up to C/O ratios of more than 25.
[0035] A C/O ratio of 26 for example means that about 90% of the
oxygen has been removed which proved to be a very good
graphite/graphene purity for several applications. For many
applications, however, poorer C/O ratios are already usable or even
preferred. Such poorer C/O ratios are e.g. a C/O ratio of at least
4, but preferably at most 13, such as a C/O ratio of about 7. For
other applications, where high C/O ratios are preferred such C/O
ratio is at least 13.5, preferably at least 20, more preferred at
least 25.
[0036] In one embodiment of the invention, the dispersion is an
aqueous dispersion. Such aqueous dispersion can be produced in a
wide pH range of 4.5 to 14 without need for a dispersant although
dispersants or other adjuvants and/or additives may be added to
further improve the features of the dispersion or a layer formed
thereof by graphene deposition.
[0037] Surprisingly, the inventors found that the parameters of the
inventive method can be varied in a broad range without affecting
the quality of the colloidal graphene dispersion.
[0038] A colloidal graphene oxide dispersion suitable as starting
material can be prepared by stirring graphite oxide powder in the
desired solvent, e.g. water, until the colloidal dispersion appears
clear to the eye. If no clear dispersion is obtained after 1 to 5
hours, the dispersion may be centrifuged to remove undissolved
impurities. The time needed may be shortened by the addition of
minimal amounts of ammonia. Such amounts usually are below a level
that would alter the pH obtained in the absence of ammonia.
[0039] It is also possible to accelerate dissolution by using
ultrasonic treatment. However, such treatment reduces the size of
the graphene oxide layers and therewith of the graphene layers.
[0040] The dispersion is then heated. A temperature of at least
about 150.degree. C. is presently assumed to be necessary for
efficient reduction, in particular for aqueous dispersions. More
than 300.degree. C. may further improve the C/O ratio, however this
is unnecessary for hitherto envisaged applications.
[0041] The heating speed is uncritical. A reaction time at the
desired temperature of less than 5 hours has been found to be
sufficient. Usually already about 1 hour at the desired temperature
is sufficient for aqueous dispersions. In the case of other
solvents adaptation of the parameters such as temperature and time
may be advantageous.
[0042] Suitably the high temperature reactions with low boiling
solvents such as water are performed in an autoclave. Neither
higher pressure than the pressure generated by the solvent is
needed nor a specific, e.g. inert, atmosphere.
[0043] Solvents other than water that may be used are polar protic
but also polar aprotic solvents such as acetonitrile or
formamide.
[0044] Also a polar solvents are suitable continuous phases for
colloidal graphene dispersions. However, while the graphite oxide
powder is readily dispersible in polar solvents, it is hardly or
not wettable in apolar solvents. Thus, for producing a colloidal
graphene dispersion in apolar solvents, the graphite oxide is
dispersed in a polar solvent and then mixed with an apolar solvent,
wherein the mixture of said polar and said apolar solvent has to be
miscible at high temperatures although not at room temperature.
Such mixture is then subjected to high temperature in an autoclave
until the reduction to graphene took place. Then the polar solvent
is removed, e.g. by decanting the upper phase or releasing the
lower phase from a separation funnel. Solvent systems suitable for
this method are known to the skilled person as TMS systems
(Temperature Depending Multicomponent Solvent System).
[0045] The high thermal conductivity, the unusual electrical
features and the high mechanical stability of graphene that is
similar to or even better than the one of carbon nanotubes make
graphene a much promising component of composite materials.
[0046] The colloidal graphene dispersions of the present invention
may be used for coating substrates such as nanoparticles that may
be used as electrode materials in rechargeable lithium
batteries.
[0047] If for chemical and/or technical reasons it is impossible to
produce a graphene/graphite comprising composit starting from the
inventive colloidal graphene dispersion, then a graphite oxide or
graphene oxide comprising composite material may be produced and
this material may then be reduced with the thermal reduction method
of the present invention.
[0048] It has also been found that graphite oxide powder can
directly be subjected to heat in a not drying environment, e.g. as
indicated above in an autoclave, resulting in a multi-graphene,
i.e. a carbonateous material with larger interspace between two
crystal lattice planes, i.e. between two graphene layers, and
therefore with improved intercalating properties compared to
grahite. Without wanting to be bound by any theory, it is assumed
that the water present between the graphene oxide layers prevents
the formation of "usual" graphite upon thermal reduction under not
drying conditions.
[0049] Such multi-graphenes in dispersed or isolated form may
either be produced directly or via precipitation from colloidal
graphene dispersions, and they have an inter-planar distance of
greater than 3.35 .ANG., preferably greater than 3.40 .ANG., or
greater than 3.50 .ANG., or greater than 3.60 .ANG., e.g. 3.55
.ANG. or 3.68 .ANG..
[0050] Under milder reaction conditions than used for
graphene/multi-graphene production with high C/O ratio, e.g.
140.degree. C. for 5 hours in an autoclave, a graphite based
material may be obtained having a much larger inter-planar
distance, e.g. of about 4.6 .ANG.. Such a material, although having
a "poor" C/O ratio was found to nevertheless have sufficient
conductivity to be used as electrode material. Dependent on the C/O
ratio, the material may be loaded with Li.sup.+ ions and used as
cathode material (graphite oxide like carbonaceous material) or it
can be used as anode material with the ability to take up Li.sup.+
ions in the inter-planar space. For example graphite oxide with the
chemical formula C.sub.8O.sub.4H.sub.2 may be treated with LiOH to
give (in case of optimal exchange) C.sub.8O.sub.4Li.sub.2.
[0051] Electrodes may be produced by coating a conductor such as an
aluminum foil with the carbonaceous materials of the present
invention.
[0052] A further benefit of the present invention is that the
reduction of graphite oxide or graphene oxide composite materials
or coatings may be performed in the presence of materials that
treated with the methods of the state of the art would be
destroyed, either by the chemical reaction or by the high
temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
[0053] The invention will be better understood and objects other
than those set forth above will become apparent when consideration
is given to the following detailed description thereof. Such
description makes reference to the annexed drawings, wherein:
[0054] FIG. 1 is a cryo-transmission electron microscope image of a
graphene dispersion
[0055] FIG. 2 is the powder diffractogram of multi-graphene
precipitated from colloidal graphene dispersion showing reflexes
similar to those of turbostratic graphite but with inter-planar
distances of 3.55 .ANG..
[0056] FIG. 3 is a powder diffractogram of a multi-graphene showing
that the reflexes are similar to those of turbostratic graphite but
with inter-planar distances of 3.68 .ANG..
[0057] FIG. 4 shows an X-ray diffraction diagram in Brag-Brentano
geometry indicating that during the coating with graphene oxide,
the graphene oxide platelets during drying were deposited in a
horizontal manner and formed the graphite oxide layer wise.
[0058] FIG. 5 shows a gold-(multi-)graphene composite that has been
obtained from colloidally dispersed gold and the inventive
colloidal graphene dispersion by co-precipitation.
MODES FOR CARRYING OUT THE INVENTION
[0059] Colloidal graphene dispersions can be readily produced by
thermal reduction of colloidal graphene oxide dispersions.
[0060] The production method for the graphene oxide powder starting
material is not critical. A suitable improved Brodie method has
been described by Boehm et al. [8]. Starting from powdered graphite
oxide, first a dispersion in the desired solvent or solvent system
is produced.
[0061] Such colloidal graphene oxide dispersion can be readily
obtained by stirring graphite oxide powder in a graphite oxide
wetting (dispersing) solvent or solvent mixture or solvent system.
The time needed for producing the colloidal graphene oxide
dispersion can vary dependent on the solvent chosen, however the
time needed can easily be determined visually, i.e. as soon as a
clear "solution" (colloidal dispersion) is obtained, stirring can
be ended and thermal reduction can be started. If the solution
remains turbid after about 5 hours or if no reduction in turbidity
can be observed for some time, impurities may be present that
should be removed prior to starting thermal reduction, e.g. via
centrifugation or a filtration step.
[0062] Ultrasonic treatment was found to speed up the
"dissolution", however it also reduced the size of the graphene
oxide and hence the size of the graphene layers. An alternative to
ultrasonic treatment is the addition of very small amounts of
ammonia.
[0063] If the solvent has a boiling point below or close to the
desired reaction temperature, the thermal reduction is preferably
performed in an autoclave to ensure that the solvent is not
evaporated during the reduction step. In spite of a high C/O ratio,
the colloidal graphene dispersions of the present invention exhibit
good stability. Such high C/O ratio is obtained at temperatures of
about 150.degree. C. and is already excellent at about 250.degree.
C. for aqueous dispersions. Treatment at more than 300.degree. C.
might lead to even improved dispersions, however, for most
applications this is unnecessary. In addition, if deposition of
graphene layers on substrates is desired during the reduction step,
low temperatures are much favored for temperature sensitive
substrates.
[0064] If for chemical and/or technical reasons it is impossible to
produce a graphene/multi-graphene comprising composit starting from
the inventive colloidal graphene dispersion, then a multi-graphene
oxide or graphene oxide comprising composite material may be
produced and this material may then be reduced with the thermal
reduction method of the present invention.
[0065] The invention is now further described by means of some
examples. In these examples an autoclave with a
polytetrafluoroethylen (Teflon.RTM.) insert was used.
Example 1
Preparation of Graphite Oxide
[0066] Graphite oxide was prepared according to the well known
method by Brodie as modified by Boehm et al. [8].
[0067] 10 g graphite were thoroughly mixed with 85 g sodium
perchlorate powder. The mixture was cooled to approx.-20.degree. C.
using an ice sodium chloride mixture and then slowly stirred with
an efficient stirrer. Then 60 ml fuming nitric acid were very
slowly added. The viscous green mass was stirred for an additional
30 minutes at room temperature. The mixture was left over night
without agitation and then slowly heated to 60.degree. C. for 10
hours. Then 2 liters of water were added to the reaction product,
the mixture was filtered and once washed with diluted hydrochloric
acid and at least twice, each time with 2 liters of water. After
filtration, the obtained mass was freeze dried yielding about 14 g
of graphite oxide as a very fluffy ivory colored powder.
[0068] Based on the elemental analysis of the graphite oxide the
chemical formula C.sub.8O.sub.4H.sub.1.7 results. After subtraction
of hydrogen as water the formula O.sub.8O.sub.3.2 is obtained with
a C/O ratio of 2.5. Using X-ray diffraction analysis it could be
shown that the inter-planar distance of 3.35 .ANG. in graphite was
enlarged to 6.1 .ANG. in dry graphite oxide.
Example 2
Preparation of a Colloidal Graphene Oxide Dispersion
[0069] 100 mg of the graphite oxide obtained as described in
Example 1 were added to 100 ml of deionized water, thoroughly
stirred for 12 hours and then left in an ultrasonic bath for 1
hour. The such obtained colloidal dispersion of graphite oxide
(further on referred to as graphene oxide), was then reacted to
colloidal graphene dispersion (see below).
[0070] The colloidal graphene oxide dispersion obtained by
dispersing graphite oxide in water was optically clear to the naked
eye and even in the light microskope at 1000 fold magnification,
free of particles and had a pH of about 5. Using a laser, the
resulting Tyndall effect showed that the graphite oxide resulted in
a colloidal dispersion.
[0071] If such dispersion is diluted and then applied to a suitable
sample holder, scanning force microscopy reveals that the colloidal
dispersion consists of single layers of oxidized graphene, i.e.
graphene oxide.
Example 3
Preparation of a Coarse Multi-Graphene Oxide Dispersion
[0072] 1 g of the graphite oxide obtained as described in Example 1
were added to 100 ml of deionized water acidified with hydrochloric
acid to a pH of about 4. After stirring for 1 hour, the obtained
coarse dispersion of multi-graphene oxide, was suitable for further
reaction to a dispersion of multi-graphene (see below).
Example 4
Preparation of Colloidal Graphene Dispersion and Multi-Graphene
[0073] The colloidal graphene oxide dispersion of Example 2 was
placed in an autoclave and heated at a temperature of 170.degree.
C. for 5 hours. During this treatment the single layers of the
graphene oxide were reduced to graphene resulting in a colloidal
dispersion of graphene in water.
[0074] The resulting colloidal graphene dispersion was deeply black
and had a pH of about 5. To the naked eye and even in the light
microscope at 1000 fold magnification, the dispersion was free of
visible particles and--as the colloidal graphene oxide
dispersion--showed the Tyndall effect of a colloidal
dispersion.
[0075] Using image giving methods such as scanning force microscopy
and transmission electron microscopy it could be shown to be a
colloidal dispersion. The single layers are e.g. clearly
recognizable in a cryo-transmission electron microscope image (FIG.
1).
[0076] The lateral diameter of the graphene layers was in the range
of the lateral diameter of the starting material and thus was
dependent of the diameter of the single graphene layers in the
graphite prior to its oxidation to graphite oxide. By evaporation
of the water from the dispersion the graphene could be precipitated
as multi-graphene. Residual water was removed by drying in vacuum
and the product was subjected to different tests.
[0077] The powder diffractogram (FIG. 2) showed reflexes similar to
those of turbostratic graphite with inter-planar distances of 3.55
.ANG..
[0078] Elemental analysis of the material gave the chemical formula
C.sub.8O.sub.0.65H.sub.0.5. After subtraction of hydrogen as water
the formula was O.sub.8O.sub.0.4 resulting in a C/O ratio of 20.
Thus, 88% of the oxygen have been removed from the graphene oxide.
X-ray diffraction and elemental analysis clearly characterized the
material as multi-graphene and thus the colloid as single graphene
layers in pure water that is longtime stable without additives, at
present for more than one year.
Example 5
Preparation of Multi-Graphene
[0079] The dispersion of multi-graphene oxide of Example 3 was
placed in an autoclave and heated for 5 hours at a temperature of
200.degree. C. By this procedure, the multi-graphene oxide was
reduced to multi-graphene. The suspension was then filtered and the
obtained multi-graphene was dried.
[0080] Due to the pH of about 4 in the dispersion medium, the
graphite oxide was not divided into its single layers but merely to
an enlarged inter-planar distance of up to about 11 .ANG. (a
multi-graphene oxide).
[0081] If graphite oxide powder is reduced according to thermal
treatment methods of the state of the art, up to about 200.degree.
C. water is removed and the inter-planar distance diminishes to
about 4.4 .ANG. while about 65% of the oxygen are removed from the
graphite oxide. Temperature increase up to 1000.degree. C. leads to
further diminishing of the inter-planar distance to 3.38 .ANG.
while about 90% of the oxygen are removed.
[0082] If the thermal reduction of a dispersion of multi-graphene
oxide was performed in the autoclave according to the present
invention at a temperature of about 200.degree. C., the inventive
kind of reduction lead to a reduction of the oxygen content of
about 90% (similar to a state of the art treatment at 1000.degree.
C.) but the inter-planar distance remained at 3.68 .ANG. instead of
the formerly found 3.38 .ANG.. For the inventive treatment at
200.degree. C. and more, the inventors assumed that under the
thereby generated conditions, such as high pressure, the water
between the layers of the multi-graphene oxide/multi-graphene
cannot leave or only with difficulties, resulting in a larger
inter-planar distance.
[0083] In a recently published article from Nethravathi and
Rajamathi, they reported a black precipitate of aggregated graphite
layers [1,1]. In contrast to this finding, the inventive method
resulted in a stable aqueous dispersion wherein the graphene layers
were not fully separated but more distant from each other than in
graphite.
[0084] The different result achieved by the present inventors is
assumed to be due to the fact that they ensured that all graphite
oxide or graphene oxide, respectively, is colloidally dispersed,
i.e. that no larger particles remain that might act as nuclei for
graphene precipitation. Although Nethravathi and Rajamathi state to
have started from a colloidal dispersion, the fact that they used
several solvents that are unsuitable to colloidally disperse the
graphite oxide, the conclusion that may be deduced thereof is that
also the aqueous dispersions were not treated sufficiently long to
ensure the total removal of all precipitation and agglomeration
favoring particles.
[0085] From the elemental analysis of the material the chemical
formula C.sub.8O.sub.0.5H.sub.0.3 was deduced and--corrected for
remaining water--the formula O.sub.8O.sub.0.35. The C/O ratio was
above 22. The powder diffractogram in FIG. 3 shows reflexes that
are similar to those of turbostratic graphite but with inter-planar
distances of 3.68 .ANG.. X-ray diffraction and elemental analysis
unambiguously characterize this multi-graphene material as closely
related to turbostratic graphite.
[0086] Under milder reaction conditions, e.g. 140.degree. C. for 5
hours in an autoclave, a graphite based (multi-graphene) material
was obtained having a large inter-planar distance of about 4.6
.ANG.. This material had a poor C/O ratio but was nevertheless
found to have sufficient conductivity to be used as electrode
material.
Example 6
Preparation of Graphene Coatings Through Thermal Reduction of
Graphene Oxide Coatings
[0087] If for chemical and/or technical reasons it is impossible to
produce a graphene/graphite comprising composite starting from the
inventive colloidal graphene dispersion, then a multi-graphene
oxide or graphene oxide comprising composite material may be
produced and this material may then be reduced with the thermal
reduction method of the present invention.
[0088] For example a colloidal graphene oxide dispersion was
produced as described in Example 2 above. This colloidal dispersion
was then applied to a quartz plate as a thin layer. The quartz
plate was then immersed into acidic water of pH about 4 (acidified
with hydrochloric acid) and the dispersion was then treated in an
autoclave at 200.degree. C. for 5 hours to reduce the graphene
oxide layers and to consequently coat the quartz plate with
graphene or multi-graphene.
[0089] During the coating with graphene oxide, the graphene oxide
platelets were deposited during drying in a horizontal manner and
formed the multi-graphene oxide layer wise. This could be shown
with X-ray diffraction in Brag-Brentano geometry (see FIG. 4). Only
the 001 reflection resulting from reflection at the single graphene
oxide layers could be seen. The inter-planar distance was 6.4
.ANG.. The 100 and 110 reflections cannot be seen as it is the case
with powder samples. After reduction, the layered structure was
also found in the so obtained multi-graphene. In the diffractogram
again only the 002 reflection of the graphite could be seen. The
inter-plane distance was 3.65 .ANG..
[0090] The elemental analysis of the material gave the chemical
formula O.sub.8O.sub.0.5H.sub.0.3, after subtraction of hydrogen as
water O.sub.8O.sub.0.3, and therewith a C/O ratio of above 22.
[0091] The X-ray diffraction and the elemental analysis clearly
characterized the material as multi-graphene.
Example 7
Preparation of a Gold-Graphene/Multi-Graphene Composit
[0092] A composite material has been obtained from colloidally
dispersed gold and the colloidal graphene dispersion of Example 4
by co-precipitation. Co-precipitation was induced by adding a very
small amount of an electrolyte such as sodium chloride. FIG. 5
shows such a gold-(multi-) graphene composit.
Possible Uses for the Colloidal Graphene Dispersions of the Present
Invention:
[0093] Graphene single and double layers show semi-metallic
features with good electrical conductivity that is almost
temperature independent. Single graphene layers are interesting for
fundamental electronic investigations and novel nano-electronic
applications. For example the Quantum-Hall Effect at room
temperature and further magneto-electrical as well as optical
features may be observed [10]. Due to these features a lot of
research was started on graphene by primarily practically as well
as primarily theoretically interested people. Hitherto only few
methods for the preparation of single graphene layers and their
deposition on suitable carriers exist. Therefore the investigations
that may be performed are limited. The known methods are very time
consuming and have little outcome of suitable samples.
[0094] Contrary thereto, the inventive method allows the production
of large quantities of graphene layers dispersed in pure water or
other solvent (see above). No additives or reducing agents are
needed. The C/O ratio can reach values above 25. From such a pure
colloidal dispersion the single graphene layers can purposefully be
deposited, e.g. between two electrodes via dielectrophoresis and
subjected to specific investigations what is hardly--if at
all--possible by means of e.g. the Scotch-Tape-Method.
[0095] By suitable methods layers and coatings can be produced from
the colloidal graphene dispersion. Dependent on the method these
can be few nanometers thin, i.e. single layers, they may be
transparent and they may for example be used as replacement for
indium tin oxide in organic light-emitting diodes, graphene
transistors, or as thin-film solar cells. But also thicker layers
may be produced such as macroscopic layers. Such macroscopic layers
are e.g. multi-graphene foils and membranes that find application
e.g. in electrical engineering and in desalinization. It is
possible to prepare layers or coatings such that an orientation and
therewith an anisotropic material results. This is also a very good
and simple alternative method to the production of highly oriented
pyrolytic graphite (HOPG).
[0096] The high thermal conductivity, the unusual electronic
features and the high mechanical and chemical stability of graphene
that exceeds those of carbon nanotubes, make graphene a much
promising material for composite materials.
[0097] From colloidal graphene dispersions in various solvents
composite materials with very homogeneous distribution of the
materials can readily be produced. The advantage of a composite
material from a colloidal graphene dispersion is especially high if
the further materials of the composite (besides of the graphene)
can also be applied in colloidally dispersed or at least in
nanodispersed form. For example a tin-(multi-)graphene composit or
a silicon-(multi-)graphene composit are much promising materials
for respective batteries, in particular if tin and silicon are also
finely divided in the solvent. In FIG. 5 a gold-(multi-)graphene
composite is shown that has been obtained from colloidally
dispersed gold and the colloidal graphene dispersion by
co-precipitation (see Example 7).
[0098] Another mode of application is the coating of a template of
any shape with the colloidal graphene dispersion of the present
invention. As many coatings as desired may be applied to the
surface of the template and then the graphene/multi-graphene
precipitated therefrom by e.g. evaporation of the solvent. As soon
as the multi-graphene layer has the desired thickness, the template
may also be removed leaving a shaped carbon product with any
desired form and wall thickness, wherein the arrangement of the
graphene layers may be anisotropic.
[0099] An envisaged new application is the production of conducting
glasses wherein the colloidal graphene dispersion is used as
additive in the scope of a sol-gel-process performed by adding the
dispersion to the sol or gel (e.g. water-glass).
Possible Uses for the Multi-Graphene Product of the Present
Invention:
[0100] Under suitable reaction conditions, e.g. 140.degree. C. for
5 hours in an autoclave, a carbonaceous material (multi-graphene)
is obtained having a large inter-planar distance of about 4.6 .ANG.
and with sufficient conductivity to be used as electrode material.
Carbonaceous materials with large interplanar distances have many
advantages as electrode materials in rechargeable batteries such as
easy intercalation/deintercalation of ions into/from the carbon
compound.
Possible Uses for the Multi-Graphene Layers of the Present
Invention:
[0101] The multi-graphene layers/coatings that may e.g. be produced
according to Example 6 have applications as membranes, anisotropic
conductors and super condensers.
Possible Further Uses of Products of the Present Invention:
[0102] Due to the possibility to produce carbonaceous materials
with almost every C/O ratio, and on almost every substrate, such
materials are also very suitable to produce condensers. For such
applications, several (e.g. about 40) layers are deposited on a
plastics foil such as a polyethylene or polycarbonate foil. The
easy variability of the C/O ratio also favors the use of the
inventive carbonaceous materials as electrode material in
rechargeable lithium ion batteries.
[0103] Such electrodes for rechargeable lithium ion batteries may
be prepared by a method comprising the step of providing a
multi-graphene and coating a conductor with said multi-graphene,
optionally in the presence of a binder.
[0104] While there are shown and described presently preferred
embodiments of the invention, it is to be distinctly understood
that the invention is not limited thereto but may be otherwise
variously embodied and practiced within the scope of the following
claims.
LITERATURE
[0105] [1] Valerie C. Moore et al.; Nano Letters; 2003; 3;
1379-1382 [0106] [2] Weijie Huang et al.; Nano Letters; 2003; 3;
565-568 [0107] [3] Dan Li et al.; Nature Nanotechnology; 2008; 3;
101-105 [0108] [4] Patent application no. WO 2008/048295 [0109] [5]
Ulrich Homann et al.; Zeitschrift fur anorganische und allgemeine
Chemie; 1937; 234; 311-336 [0110] [6] H. P. Boehm et al.;
Zeitschrift fur Naturforschung; 1962; 17b; 150-153 [0111] [7] H. P.
Boehm et al.; Zeitschrift fur anorganische und allgemeine Chemie;
1965; 335; 74-79 [0112] [8] H. P. Boehm et al.; Annalen der Chemie;
1965; 691; 1-8 [0113] [9] K. S. Kim et al., Nature Letters; 2009,
457, 706-710 [0114] [10] A. K. Geim et al.; Nature Materials; 2007;
6; 183-191 [0115] [11] C. Nethravathi, Michael Rajamathi; Carbon;
2008; 46; 1994
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